A tubular camshaft is recently used in an internal combustion engine to meet the requirement of weight-reduction. The inside capacity of a tubular camshaft is usually too large to be used as an oil passage. If it were used as an oil passage without being modified, the sliding surface would scuff due to poor lubrication in the starting period, because too much time would be needed for the oil to fill the capacity prior to attending to the sliding surface. Therefore, it has been proposed to reduce the inside capacity of the tubular camshaft.
The camshaft with a modified inside capacity is known by JP A 55-132417 and JP U 57-75105. The former has double inner and outer cylindrical fillers coaxially fitted in a tubular shaft, the inner filler being oil-permeable and disposed inside the outer filler which is formed with an oil passage. The latter has a single filler of resin or foamed material fitted in a tubular shaft, the filler being formed with an oil passage by machining. However, the known camshafts are disadvantageously costly in production. The former needs a couple of fillers and expensive machining works to provide an oil passage in the outer filler. The latter needs a special die or core to mold the filler and costly machining works to form an oil passage after the filler is molded.
The present invention is intended to resolve the problem as described above and provide a tubular camshaft with a lubricating-oil supplying function that can be easily manufactured without expensive machining works. It also provides a camshaft wherein the sliding surface is well lubricated and protected against scuffing in the starting period.